2Charlie, I’m afraid there are just too many things that could go amiss to try to answer with that error message alone. (But if anyone else may immediately recognize it, that would be great.)
I will say also that there are many moving parts you have to get just write, in CFB, in CF, perhaps in your web server, and perhaps in the network between them. And too many and not trivial to elaborate even briefly here, or go over with you one by one to make sure all is setup correctly.
Instead, I would point you to a presentation I did, “Setting up ColdFusion Builder Step Debugging”:
where I do address common challenges. And note that I have both the session recording and a PDF of the notes from the talk offered there.
I also address troubleshooting it in the chapter I did in the CFWACK book for CF9, which chapter is available online:
And besides the troubleshooting, it may offer some useful insights into setting up and using the debugger. Let us know how you get on. (I’ll add finally that if this is time-critical and you just want the problem solved, I can also try to help via my consulting services. I doubt it would take more than an hour to resolve whatever is amiss, and I have a satisfaction guarantee so you won’t pay for time you don’t find valuable. More at carehart.org/consulting.)